1. Ask your contracting officer for any compliance guidelines or check online guidelines published by the relevant compliance agency e.g. FAR for US Gov't DoD.
2. Whenever the group reaches a particular milestone (e.g. kick-off or close a project), when they need a group team-building moment or for a recognized national holiday it should be appropriated to take everyone out to dinner or bring in pizza, etc.
Where you may create the appearance of impropriety is when you do things for specific individuals rather than the group.